Fatigue is one of the most common and distressing side effects of breast cancer treatment and may persist for months or years after successful treatment completion. Approximately one-third of breast cancer survivors report moderate to severe symptoms of fatigue, which has a negative impact on all aspects of quality of life. Although the prevalence and impact of cancer-related fatigue has now been well established, very little is known about predictors and mechanisms for the development and persistence of fatigue post-treatment. Accordingly, the primary goal of this prospective, longitudinal study is to identify biological and psychological risk factors for post-treatment fatigue, with intensive evaluation of mechanisms, in women newly diagnosed with early stage breast cancer. In particular, this application focuses on vulnerability factors that increase risk for inflammatory processes given evidence suggesting an inflammatory basis for cancer-related fatigue. Specific aims are to: 1) determine whether inflammatory risk genes, hypothalamic-pituitary-adrenal axis dysregulation, and body mass index at treatment onset predict post-treatment fatigue in breast cancer patients assessed longitudinally for 18 months after treatment completion; 2) evaluate whether history of depression and early life stress at treatment onset predict post-treatment fatigue in breast cancer patients assessed longitudinally for 18 months after treatment onset; 3) investigate the contribution of measured proinflammatory cytokine activity to the association between biological and psychological risk factors and post-treatment fatigue. We will recruit 360 women with newly-diagnosed, early-stage breast cancer before initiation of treatment with radiation, chemotherapy, trastuzumab or endocrine therapy. At baseline, participants will complete self-report questionnaires, diagnostic interview for depression, blood draw for cytokine gene polymorphisms and markers of inflammation, and saliva samples for diurnal cortisol slope. Follow-up assessments conducted at treatment completion and at 6, 12, and 18 months post-treatment will determine the trajectory of post-treatment fatigue and associated changes in inflammatory processes. This research is a critical next step in the early identification of patients who are at risk for persistent fatigue as a long term side effect of cancer treatment and for the development and implementation of targeted interventions to prevent and treat this disabling symptom.